1. Field of the Invention
The invention relates to a fuel cell system that includes a fuel cell that generates electricity using a fuel gas and an oxidizing gas, a vehicle-driving motor inverter, an accessory motor inverter, and a converter that supplies output voltage of the fuel cell to the vehicle-driving motor inverter and the accessory motor inverter after raising the voltage.
2. Description of the Related Art
A fuel cell system that is mounted in a fuel cell vehicle or the like, and that drives a vehicle-driving motor provided as a drive source of the vehicle by supplying the motor with electric power from a fuel cell stack is known. The fuel cell stack generates electricity using a fuel gas and an oxidizing gas.
Besides, Japanese Patent Application Publication No. 2004-146118 (JP-A-2004-146118) discloses a fuel cell system that includes a fuel cell stack that supplies electric power to a vehicle-driving motor and an air compressor for supplying an oxidant to the fuel cell stack, a secondary battery, voltage conversion means that is connected to the fuel cell stack and that performs an operation of extracting electric power from the secondary battery and boosting the voltage or that performs an operation of supplying electric power to the secondary battery after lowering the voltage of the electric power, and control means. The control means, when activating the fuel cell stack, controls the voltage conversion means so that the voltage conversion means performs a voltage-boosting operation of boosting the electric potential at a connection point between the air compressor side of the fuel cell stack and the voltage conversion means to a voltage that is higher than the open-circuit voltage of the fuel cell stack, and therefore electric power of the secondary battery is supplied to the air compressor, and, after operating the air compressor for a certain time, performs a voltage-lowering operation of lowering the voltage from the level that is higher than the open-circuit voltage, so that electric power from the fuel, cell stack is supplied to the air compressor.
Besides, a fuel cell system capable of supplying a vehicle-driving motor and an accessory, such as an air compressor for supplying an oxidizing gas to a fuel cell stack, with electric power from the fuel cell stack, via a shared converter that performs a voltage-converting operation, is also conceivable. In this case, the converter is provided between the fuel cell stack and both the vehicle-driving motor and accessories. According to this construction, the voltage of the fuel cell stack can be raised by a converter, and then can be supplied to the vehicle-driving motor and the air compressor.
In the fuel cell system in which the converter is provided between the fuel cell stack and both the vehicle-driving motor and accessories, the input voltages to the vehicle-driving motor and the input voltage to the accessories, such as the air compressor and the like, are the same. The input voltage to the accessories herein is actually the input voltage to the accessory motors for driving accessories. In some other cases, the service voltage range of the vehicle-driving motor is designed as being a certain range, and the voltage of the accessory motors is designed as being the lowermost limit of the service voltage range of the vehicle-driving motor. Since the service voltage of the accessory motors changes according to the actual service voltage of the vehicle-driving motor, accessory motors in actual use are sometimes operated at a voltage that is higher than the designed voltage, which becomes a factor of deterioration of efficiency. Therefore, it is desired that the designed values of the service voltages of the accessory motors be close to their actual service voltages.
On another hand, if the service voltage of the accessory motor is designed as being a voltage that is higher than the foregoing lowermost limit of the service voltage of the vehicle-driving motor, the input voltage to the vehicle-driving motor is boosted in response to a demand from an accessory motor every time there is a transition response during the operation of the fuel cell stack, for example, every time the fuel cell vehicle undergoes acceleration or the like. Due to such frequent performance of the voltage boost operation by the converter, the loss may possibly become large, and the efficiency of the system as a whole may possibly deteriorate.